Coal, wood and other biomass can be converted to liquid products by various thermochemical and biochemical processes. The liquids from coal and biomass conversion reactions generally contain hydroxyaromatic hydrocarbon compounds due to the high oxygen content of most biomass (40-50 wt % on a dry basis) and as such the liquids are not true hydrocarbons. This limits their ability to replace petroleum-derived fuels.
Oil produced by high-pressure catalytic liquefaction of wood and other biomass has been widely reported as being phenolic, with both single and multiple ring phenols present. The relative amounts of each are a function of the processing severity and the type of biomass. Oils produced by flash pyrolysis of biomass are even more highly oxygenated and are thermally unstable when subjected to hydrotreating. With additional catalytic treating, pyrolyzates can be converted to oils chemically similar to those produced by high-pressure, catalytic liquefaction, such as disclosed in our patent application Ser. No. 033,281 filed Apr. 2, 1987 and entitled Process for Upgrading Biomass Pyrolyzates, the disclosure of which is hereby incorporated by reference.
Similarly, certain coal conversion processes produce phenolic-type oils as a portion of the product or as a byproduct. Coal-derived oils that are primarily phenolic include low-temperature coal tar such as that produced as a byproduct from fixed bed coal gasifiers and coal liquids from low severity hydrogenation processes.
Another raw oil material which contains single and multiple ring compounds is derived from the black liquor byproduct of chemical pulping processes. Liquid-phase treatment of black liquors, from alkaline pulping, at 300.degree.-350.degree. C. in a reducing atmosphere results in the formation of a phenolic-type oil product, which separates out from an aqueous phase containing the inorganic constituents. Such a procedure is described by McKeough et al. in "Oil Production by High-Pressure Thermal Treatment of Black Liquors," Oil Production from Black Liquors, ACS Symposium Series #376, pp. 104-111 (1988).
The production of actual hydrocarbon fuels from the products of the various biomass and coal and wood conversion processes has remained an elusive goal despite decades of research. Processing research has included a number of techniques, none of which has been totally successful. Biochemical conversion of cellulosic materials to ethanol is probably one of the most advanced techniques. Although the ethanol can be blended into gasoline, it is not a true hydrocarbon and requires engine modifications to be used directly. The recovery of plant-produced hydrocarbons such as seed oils or latex have also been investigated as diesel oil substitutes, but remain experimental in nature. Hydrocarbons, albeit high molecular weight and of a polycyclic aromatic nature, can also be produced as a small byproduct of high-temperature biomass gasification.
Another approach has been single step processes of various biomass and coal and wood conversion products which have had limited success in producing substantial quantities of highly aromatic hydrocarbons. Since some of the heavy fossil fuels such as heavy oils, shale, and coal liquids have high concentrations of organo-oxygen compounds, hydrodeoxygenation in the presence of standard hydroprocessing catalysts, such as Co-Mo/Al.sub.2 O.sub.3 and Ni-Mo/Al.sub.2 O.sub.3, has been investigated. This single step process has not been that successful in producing highly aromatic gasoline because at low temperature the hydrodeoxygenation reaction is suppressed in favor of the hydrogenation reaction of existing aromatics, which is not only undesirable, but also a waste of hydrogen. Furthermore, hydrodeoxygenation reactions still leave multiple-ring aromatic compounds as undesirable end products.
Hydrocracking proceeds by a free radical mechanism when a non-acidic alumina supported catalyst is used. Here, too, results in producing substantial quantities of highly aromatic gasoline have been limited. Existing aromatic compounds become saturated, which is undesirable and a waste of hydrogen. Moreover, there is no convincing evidence of cracking multi-ring aromatics. Generally, although exhibiting some success, single step processes have not proven to be optimal for producing highly aromatic fuel.
Recent developments in direct liquefaction of biomass have focused on two processing environments: 1) high-pressure, catalytic systems requiring extended residence time at 300.degree. C. to 400.degree. C. and 2) flash pyrolysis systems which operate at higher temperatures (approximately 500.degree. C.) and atmospheric pressure with short resident times (&lt;1 second). Neither of these systems can produce hydrocarbons directly in any significant yield.